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I am getting this message when building a application but do not have
any idea what is causing the problem. The output that goes with it is
just the normal output of the g++ command. I have tried the --verbose
cli flag to the g++ command but this gives no further information.
Any ideas on how I can get more information from ld and collect2 to see
what the problem is?
USUALLY this means that you have unresolved symbols or missing input files. Do you not get ANY indication of a problem? I suspect the verbose flag will just confuse you.
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Mats
If there are unresolved symbols or missing input files, I think compiler should throw error(s) at compiling-time, not linking-time
Unresolved symbols are linking errors. Meaning the linker was unable to find the symbol and thus can't generate any code for it.
thanks. but what do you mean "missing input files"
Usually, you did not add a .c or .cpp file to your project. The compiler will declarations of your functions and think they exist, but the linker won't find them because the compiler didn't compile the code for the functions.
Thank you, Elysia. One more question:
You mentioned that this error is thrown when I use a symbol/function but never declare/define it. Now if I declare it but never define it, will that error be thrown?
If a symbol is not declared, the compiler will complain.
If a symbol is declared but never defined, the linker will complain.
Let me summarize your points about a symbol's declaration, definition, and using:
If a symbol is not declared, whether you use or define it, the compiler will complain.
If a symbol is declared but never defined, whether you use it or not, the linker will complain.
If a symbol is declared and defined, whether you use it or not, the linker will NOT complain.
Yes to all three statements.
Although to your first question, there is an exception. If the function you are trying to call is defined above the function you're calling it from, you don't have to declare it. The compiler will generate code as it should.
HOWEVER, this is really bad practice, so don't do it.
Also note that this applies to normal functions, classes and variables, but not templates.
Actually, the linker won't complain if a symbol is declared but not defined, but you don't actually use it.
Kind of depends on what kind of symbol it is. I don't think it's the same for all. Like an extern global variable will create a linking error if you don't define it, I believe?
Nevertheless, it's just good practice to define everything you declare. The declarations are just there for the compiler, after all, and if you're telling the compiler they exist, then you should make sure they exist too.
Yes, it's more about for example the difference between including stdio.h, and actually using all the dozens of functions in stdio.h - most programs only use half a dozen of those functions, but you CAN use rewind, ungetc, unlink, fwprintf, etc, etc if you really want to.Quote:
Originally Posted by Elysia
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mats
Try adding -Wl,-V (exactly as I've written it) to the gcc options to turn on verbose linker output.Quote:
Originally Posted by meili100